Over the past several decades air pollution control has been a priority concern of society. In the United States primary regulatory authority over industrial source air emissions resides in the U.S. Environmental Protection Agency (“EPA”). Over the years, the EPA has increased the stringency of its air pollution control programs, both by decreasing the limits on acceptable emissions and by continually increasing the number and types of regulated pollutants. Where potentially toxic compounds are released into the atmosphere, sources are required to use the maximum available control technology (“MACT”). Mercury (Hg) is a particularly toxic substance that is dangerous to humans at very low concentrations. Mercury and its compounds are highly persistent in water and the environment and bioaccumulate or concentrate in the tissues of fish. The types of pollutants emitted from an industrial source and the technologies available to control the pollution are highly dependent on the specific industrial process in use. EPA has adopted MACT standards for the control of mercury emissions into the atmosphere associated with the manufacturing of cement.
The inventors' prior U.S. Pat. No. 7,279,039, the disclosure of which is incorporated by reference, describes an apparatus and method for reducing emissions of various types of air pollutants from cement plants. However, the approaches disclosed in their prior patent do not address the specific problems associated with mercury pollution from cement plants. In cement manufacturing mercury may be found both in the fuels and raw materials used in the process. During intense heating in the preheater tower and/or kiln which is necessary to form cement clinker, mercury and most commonly formed mercury compounds (collectively “mercury pollutants”) are vaporized and may be emitted with combustion gases. For example, elemental mercury has a substantial vapor pressure even at 95°-105° C. A fraction of the mercury entrained in the exhaust gas flow condenses on the kiln dust or raw meal under certain conditions and may be discharged with waste cement kiln dust (CKD). However, the major fraction of the mercury is emitted unimpeded from existing air pollution equipment (i.e., fabric filters) as a gas. The emitted mercury pollutants may include oxide or salts (ionic) and/or molecular vapor (elemental) depending on kiln equipment operation and design.
The current methods available for control of mercury emissions in cement production include: 1) reduction of mercury inputs, 2) capture of mercury at the point of emissions using carbon adsorption, and 3) capture in wet scrubbers. Each of these control methods has a significant economic impact on the cost of cement production through substantially higher operating costs or substantial capital cost for new equipment. Current technology used for mercury emission control include the adsorption of the oxide on activated carbon injected after the removal of kiln feed dust from the main plant gas stream. This approach involves the use of a secondary capture system after the primary particulate control device. Carbon injection rates are significant (i.e., 1 to 5 lb/100,000 acfm) and the capital cost of the secondary capture device is high (approximately $25 million). Moreover, for effective capture, the mercury must be present in the oxide form and have a predictable emission rate. In addition, the carbon to adsorb the mercury must, itself, then be treated as a waste stream requiring regeneration off-site or disposal in a suitable landfill at high additional cost.
Mercury may also be removed from the gas stream concurrent with SO2 in the wet scrubbers used in some systems. In this case, the gypsum product generated contains the mercury oxide and cannot be used as synthetic gypsum in the finish mills. Further, this approach results in the presence of mercury in the scrubber liquor discharge, requiring special wastewater treatment to remove the mercury prior to discharge.
The foregoing problems are applicable to cement manufacturing facilities including those which use a precalciner prior to feeding the meal into the pyroprocessing kiln. A typical modern, prior-art cement manufacturing facility is shown in FIG. 1. While other dry and wet cement manufacturing processes are known, the dry precalciner process depicted in FIG. 1 is now the most common and efficient.
The primary feed material, comprising a calcium-containing mineral used in manufacturing the cement, is obtained from a quarry, usually located nearby the cement plant. Typically the primary feed material is limestone, with smaller quantities of sand, clay, shale, and/or bauxite also being used. The feed materials provide the calcium, silica, aluminum and iron necessary to produce cement. However, these feed materials contain naturally occurring mercury, typically in range of 2 to 40 parts per billion (ppb), which cannot be avoided by selective mining. Likewise, the fuel (coal) and added fly ash from the coal also contain appreciable amounts of mercury. The fuel supply does not generally support changes to achieve low mercury content. The most effective raw material change is substitution of bauxite for fly ash or other alumina sources. However, this has a significant economic impact due to the low cost of fly ash and high cost of bauxite (imported) and the necessity for additional equipment changes. Moreover, the mercury content of fly ash has increased as utility boilers have changed operations to reduce mercury gas emissions by concentrating the mercury in the captured fly ash.
The quarried material is reduced in size by a crusher (not shown), and the crushed raw material is then transported to the cement plant, for example by motor or rail vehicle or by conveyor (also not shown). The proper proportions of the raw materials are then mixed and further reduced in size in a raw mill 10 to form a meal or feed material. For convenience the term “feed meal” is generally used herein to refer to the solid materials from the time they are processed in the raw mill to the time they enter the kiln. Thus, as used herein, feed meal includes the meal that has undergone precalcining.
The raw feed meal from raw mill 10 is then preheated in a preheating tower, comprising a series of vertically stacked cyclone chambers using exhaust gas from the kiln. While two such cyclone chambers (21, 22) are shown in FIG. 1, more (typically 3 or 4) may be used. Collectively these are referred to herein as the preheating tower and includes a precalciner 30. As depicted in FIG. 1, feed meal from raw mill 10 enters at the top of the preheating tower 21 and is preheated as it descends under the force of gravity.
The heated feed meal is introduced into a precalciner 30 at the bottom of the preheating tower, where the calcium carbonate (CaCO3) in the limestone (or other feed material) is converted into calcium oxide (CaO), releasing a large amount of carbon dioxide (CO2) in the process, thereby increasing the volume of the gas flow. This conversion is accomplished by heating the feed meal to high temperature—between about 900° C. and 1,000° C. The required temperature is higher than the temperature of the kiln exhaust gases, and so typically additional heat is generated in the precalciner by combustion of auxiliary fuel.
After precalcination the feed meal is introduced into a large rotary kiln 40 where it is heated to a temperature of about 1,500° C.) to form “clinker,” consisting primarily of calcium silicates. Rotary kiln 40, which may be as long as 700 feet (213 meters), is substantially horizontal, with a slight tilt sufficient for gravity-assisted transport of the materials undergoing pyroprocessing along its length. Various fuels may be used to support combustion within the kiln in order to achieve the high processing temperature that is required. The hot clinker is then discharged from the kiln into a cooling chamber 50. After being cooled, the clinker is discharged from cooling chamber 50 and ground into fine particles. Normally, a small amount of gypsum is added during this final stage of the process.
The air used for combustion in kiln 40 first flows through cooling chamber 50, where it gains heat as it cools the clinker. The hot exhaust gases from kiln 40 flow through the precalciner 30 and then to the preheating tower 21 as described. After combustion in the kiln very little oxygen remains in the exhaust gas flow, and so additional air is introduced into precalciner 30 to support combustion, again increasing the volume of gas that is ultimately discharged from the plant. After passing through preheating tower 21, the exhaust gases are routed through raw mill 10 used to grind the raw feed materials, before being discharged into the atmosphere via stack 60. Contact with the hot exhaust gases heats and dries the feed meal in the grinder. Because of the high particulate load, a baghouse or electrostatic precipitator 70 is used to remove particles from the gas flow, which are typical recycled back into and blended with the feed meal.
In FIG. 1 the movement of the solids (i.e., the feed materials, clinker, etc.) between the various processing operations is shown by solid lines, while the flow of gases is shown with dashed lines. It can thus be seen that the gas flow through the process is generally counter to the flow of the solids and, from the time the gases leave the kiln to the time they are exhausted into the atmosphere, they exchange heat with the feed meal, i.e., the gases are cooled as the feed meal is dried and heated. Thus, for example, the feed meal is progressively heated as it travels down the preheating tower from one preheating cyclone to the next, while the flue gases become successively cooler as they travel up the preheater tower.
Recent published test data on mercury emissions from preheater/precalciner cement kilns employing in-line rock grinding to produce kiln feed, show that at preheater exit conditions (i.e., temperature and oxygen) elemental mercury vapor is converted to an oxide form (at <500° C., optimum at 300°-350° C.). The various oxides of mercury (referred to herein as mercury oxides) condense into particles when the gas temperature is further reduced. Further gas temperature reduction occurs when the exhaust gas flow from the preheater tower is used to heat the feed meal in the grinder. Most of the mercury pollutants are condensed by cooling in the grinder and are recaptured in the feed meal. They are then returned with the feed meal to the preheater tower. In the preheater, the captured mercury oxides in the feed are again vaporized and re-emitted into the gas flow, resulting in a recirculating pattern that increases its concentration over time. However, when the in-line grinding mill is down, the enriched recirculating load of mercury is no longer subject to recondensation in the mill and, instead, is emitted from the process. Under these conditions, the grinding mill is bypassed and the temperature of the exhaust gas is sufficiently high (190°-230° C.) that mercury pollutants in the exhaust gas flow do not condense in the fabric filter dust layer. In contrast, when the mill is on the exhaust flow entering the fabric filter is much cooler (e.g., 95°-110° C.) such that any mercury pollutants that are not condensed and captured in the grinder will be condensed and captured in the fabric filter dust layer. Limited test data indicate that mercury emissions during mill down periods may be 3 to 15 times higher than during mill-in periods. The exact concentration of emitted mercury is a function of the mercury content of the raw materials, the ratio of mill-in to mill-down operating time, and efficiency of conversion of vapor to oxide species in the exhaust of the preheater tower. The predominate mercury pollutants emitted during the mill-in period is elemental mercury vapor and during the mill down is mercury oxide.
FIG. 2 shows an improvement to the prior art arrangement shown in FIG. 1 as set forth in the inventors' U.S. Pat. No. 7,279,039, wherein the feed meal is heated at the outset of the process to drive off volatile compounds which are then combusted. Unlike the volatile organic compounds and other pollutants described in the inventors' prior patent, which are substantially destroyed (i.e., broken down into harmless compounds) by the combustion process, mercury is not broken down and, instead, is simply reintroduced into the gas stream. Once in the gas flow it behaves in a manner similar to that described in connection with FIG. 1; i.e., the mercury oxides are formed in the preheater tower and are condensed when the gas flow is used to heat the feed meal in the grinder. Ultimately, the mercury pollutants are either emitted into the atmosphere with the plant exhaust, or recaptured in the feed meal and recirculated.